diff --git a/version.py b/version.py
index 7f5c5abd55..aa3cd674ed 100644
--- a/version.py
+++ b/version.py
@@ -18,7 +18,12 @@ LATEST_RELEASE_ANNOUNCEMENT = "https://blog.zulip.org/2019/03/01/zulip-2-0-relea
 # the major version to indicate that folks should provision in both
 # directions.
 
-# Typically, adding a dependency only requires a minor version bump, and
-# removing a dependency requires a major version bump.
+# Typically,
+# * adding a dependency only requires a minor version bump;
+# * removing a dependency requires a major version bump;
+# * upgrading a dependency requires a major version bump, unless the
+#   upgraded dependency is backwards compatible with all of our
+#   historical commits sharing the same major version, in which case a
+#   minor version bump suffices.
 
 PROVISION_VERSION = '40.0'